1. Field of the Invention
The present invention relates to an obstacle detection device using a leaky transmission path such as a leaky coaxial cable, a leaky wave guide and the like, and more particularly relates to an obstacle detection device and obstacle detection system precisely detecting the presence and position of an obstacle utilizing spread spectrum technology regardless of whether the obstacle is motionless or moving.
2. Description of the Related Art
A fallen object from a traveling vehicle, a vehicle performing an emergency stop operation or the like are considered as examples of an obstacle on ordinary roads and railroad lines. Since these obstacles in a state of being motionless on the ordinary roads and railroad lines are factors causing the occurrences of a rear end collision and a double rear end collision accident, in order to prevent these occurrences, it is also necessary to promptly detect these obstacles and perform removal operations for these obstacles.
As an obstacle detection device for detecting an obstacle on the ordinary roads and railroad lines arising from the above-described request, there are obstacle detection devices which use leakage transmission paths such as a leakage coaxial cable, a leakage wave guide and the like. A brief explanation of the configurations of these leakage transmission paths will be given hereafter. A leakage wave guide is one in which multiple slots leaking and radiating a radio wave in the longitudinal direction of a wave guide comprising a conductor are provided at suitable intervals. A leakage coaxial cable has a configuration based on a principle similar to that of the leakage wave guide. A related-art obstacle detection device using a leakage coaxial cable (hereafter, referred to as LCX) will be described below.
FIG. 6 is a block diagram showing a configuration of a related-art obstacle detection device disclosed in Japanese Laid-Open Patent Application No. 10-95338. In FIG. 6, reference numeral 1xe2x80x2 denotes a transmission LCX which is laid on one side of a road or a railroad line and provided with a plurality of slots leaking and radiating a pulse modulated signal for detection at suitable intervals in the longitudinal direction, reference numeral 2xe2x80x2 denotes a receiving LCX which is laid on the opposing side of the transmission LCX 1xe2x80x2 laid on the road or the railroad line and receives the radiated pulse signal from the transmission LCX 1xe2x80x2 by a plurality of slots provided at suitable intervals in the longitudinal direction. Matched terminations are connected at far ends of the transmission LCX 1xe2x80x2 and the receiving LCX 2xe2x80x2 opposite to a transmitter 63 and a receiver 64, respectively. Reference numeral 63 denotes a transmitter connected to one end (near end) of the transmission LCX 1xe2x80x2 and generating a pulse modulated signal for detecting an obstacle; reference numeral 64 denotes a receiver connected to one end (near end) of the receiving LCX 2xe2x80x2, which end is opposite to the near end of the transmission LCX 1xe2x80x2, and receiving the pulse modulated signal for detection from the transmission LCX 1xe2x80x2; reference numeral 65 denotes a portion of the receiver 64 operating as a low pass filter (hereafter, referred to as LPF) for extracting an envelope from a waveform of the pulse modulated signal for detection which has been received by the receiver 64; reference numeral 66 denotes a portion of the receiver 64 operating as a memory device for storing an envelope extracted from the waveform of pulse modulated signal for detection when an obstacle does not exist; and reference numeral 67 denotes a portion of the receiver 64 operating as a computing unit in which the difference between an envelope from a waveform of the pulse modulated signal for detection extracted by the LPF 65 and an envelope stored in the memory device 66 when an obstacle does not exist is found and the position of the obstacle is detected from the difference waveform.
A description will now be given of the operation according to the related art.
A signal for detecting an obstacle which has been pulse modulated in the transmitter 63 is outputted to the transmission LCX 1xe2x80x2. The inputted pulse signals in the transmission LCX 1xe2x80x2 are in turn radiated as a radio wave from the respective slots aligned in the longitudinal direction of the transmission LCX 1xe2x80x2. This radio wave enters the respective slots provided in the longitudinal direction of the receiving LCX 2xe2x80x2 opposed to the transmission LCX 1xe2x80x2, and received by the receiver 64 with a delay corresponding to the positions of the slots. When the receiver 64 receives a radio wave from the transmission LCX 1xe2x80x2, the LPF 65 in the receiver 64 extracts an envelope from a waveform of the pulse signal for detecting an obstacle received as a radio wave from the transmission LCX 1xe2x80x2 and transmits the extracted envelope to the computing unit 67. The computing unit 67 reads the envelope (reference waveform) previously measured when an obstacle does not exist from the memory device 66 whenever the LPF 65 extracts an envelope from a waveform of the received signal and finds a difference waveform between the read envelope and the envelope of the waveform of the pulse modulated signal for detection which the LPF 65 has extracted. At this time, if an obstacle exists on the road or the railroad line, which intervenes between the transmission LCX 1xe2x80x2 and the receiving LCX 2xe2x80x2, the radio wave from the transmission LCX 1xe2x80x2 is interrupted at its position. Therefore, if an obstacle exists, a reception level of a radio wave received by the receiving LCX 2xe2x80x2 from the transmission LCX 1xe2x80x2 is reduced by a certain degree, irrespective of the field intensity provided by the transmission LCX 1xe2x80x2. Owing to this, since a change corresponding to the obstacle appears in the difference waveform calculated by the computing unit 67, the presence of the obstacle can be detected.
Since the related-art obstacle detection device is configured as described above, and because the coupling loss of a signal transmitted via a LCX is large, the received S/N ratio of the pulse modulation signal that the receiver 64 receives is small and the reference waveform used during the detection of an obstacle varies, resulting in a problem that an obstacle could not be detected with the sufficient precision to satisfy reliability demands.
The above-described problem will be concretely described below.
FIGS. 7A-7H are graphical representations showing a transmitted waveform and received waveform of a related-art obstacle detection device described above. FIG. 7A shows a waveform of a pulse modulated signal for detecting an obstacle outputted to the transmitting LCX 1xe2x80x2 by the transmitter 63. FIG. 7B shows an ideal waveform of a signal received by the receiver 64 via the receiving LCX 2xe2x80x2 without considering factors such as the coupling loss and noise of LCX, FIG. 7C shows a waveform of a signal received by the receiver 64 via the receiving LCX 2xe2x80x2 in consideration of the coupling loss of LCX, FIG. 7D shows an envelope extracted from a signal waveform of FIG. 7C, FIG. 7E shows an observed waveform of a received signal by the receiver 64 via the receiving LCX 2xe2x80x2, FIG. 7F shows an envelope extracted from the waveform of FIG. 7E, FIG. 7G shows a waveform when the noise is added to a signal received by the receiver 64, and FIG. 7H shows an envelope from the signal waveform of FIG. 7G.
A signal outputted from the transmitter 63 to the receiving LCX 1xe2x80x2 is pulse modulated and shows a sinusoidal waveform as shown in FIG. 7A. This signal is radiated as a radio wave and inputted into the receiver 64 via the receiving LCX 2xe2x80x2. A radio wave from the receiving LCX 1xe2x80x2 is a waveform as shown in FIG. 7B when factors such as the coupling loss and noise of LCX are not considered; i.e. a succession of pulse modulated signals of FIG. 7A with mutual delays corresponding to respective positions of the slots.
Since the coupling loss to a signal transmitted via the LCX exists in a LCX and the coupling loss is not uniform in the longitudinal direction. When considering the coupling loss of LCX, the waveform becomes as shown in FIG. 7C wherein an amplitude is varied. Moreover, the envelope extracted from the signal waveform shown in FIG. 7C has a waveform as shown in FIG. 7D.
However, since actually the coupling loss of LCX is large, in the case where one pulse modulated signal is received by the receiver 64, the waveform is not as shown in FIG. 7C. A waveform as shown in FIG. 7E, wherein the reception level and reception SN ratio are low, is observed. When such a signal passes through the LPF 65, an envelope as shown in FIG. 7F is extracted and stored in the memory device 66.
After a signal waveform shown in FIG. 7E is observed, a waveform as shown in FIG. 7G, instead of the waveform as shown in FIG. 7C, may be observed in a state in which there is no obstacle. This happens when noise is added to the signal. owing to this, an envelope extracted from a signal waveform shown in FIG. 7G by the LPF 65 has a waveform as shown in FIG. 7H. Therefore, an envelope shown in FIG. 7F previously stored in the memory device 66 and an envelope shown in FIG. 7H have different waveforms, and if a difference calculation is performed using an envelope previously stored in the memory device 66 as the reference waveform, there may be the possibility that error detection will occur. Thus, since the reference waveform used to detect an obstacle varies depending upon the conditions at the time of measurement, there has been a problem that an obstacle could not be detected with sufficient precision to ensure reliable performance.
The variation of the reference waveform can be solved by performing integration for multiple measurements. While this approach is effective for an obstacle in a state of being motionless, since the measurement time period is longer, for example, it is not effective for a moving obstacle such as a person straying onto a railroad line and so forth. Therefore, there has been a problem that a moving obstacle could not be detected by the related-art obstacle detection devices.
Furthermore, there has been a problem that a related-art obstacle detection device is susceptible to jamming.
To describe this in further detail, given that an input electric power from the transmission LCX 1xe2x80x2 of the receiver 64 is P1 Watts (hereafter, referred to as W) and an input jamming electric power for the receiver 64 is P2 (W), a DU ratio which is an electric power ratio of a jammer with respect to the desired radio wave of the receiver 64 of the obstacle detection device is expressed by DU=10 log (P1/P2). In the case where the jam is strong and the equality of P1=P2 is satisfied, the DU ratio becomes 0 (zero) from the above-described expression, the receiver 64 falls into a state of not being capable of receiving a radio wave from the receiving LCX 1xe2x80x2.
As a problem apart from the above-described problems, there is a problem that a wireless communication device cannot be used in the detection area where an obstacle detection device detects an obstacle.
To describe this in further detail, given that a radiating electric power of a radio wave from the transmission LCX 1xe2x80x2 is P3 (W) and an input electric power of the wireless communication device in the detection area of the obstacle detection device is P4 (W), a DU ratio which is an electric power ratio of the radio wave from the transmission LCX 1xe2x80x2 with respect to the desired radio wave of the above-described wireless communication device is expressed by DU=10 log (P4/P3). In the case where the jam is strong and the equality P3=P4 is satisfied, the DU ratio becomes 0 (zero) from the above-described expression.
Moreover, since the detection of an obstacle is performed by utilizing an envelope which is an output from the LPF 65 located within the receiver 64 in a related-art obstacle detection device, it is not possible to discriminate between slots of the transmission LCXxe2x80x2 to determine a slot responsible for radiating the pulse-modulated signal. Therefore, there has been a problem that precise detection of an obstacle""s position could not be performed.
Moreover, in an obstacle detection system configured by providing multiple related-art obstacle detection devices, there has been a problem that interference between pulse modulated signals occurs where the obstacle detection devices were adjacent to each other, resulting in a failure to detect the obstacle.
Furthermore, in an obstacle detection system configured by providing a plurality of related-art obstacle detection devices, it has been required-that an electric power source supplying device for supplying the electric power source to the obstacle detection devices which constitute the system were provided in every obstacle detection device. The problem of increases in manufacturing costs has resulted due to the necessity to provide for installation space for electricity supply devices.
Finally, there has been a problem that information could not be precisely transmitted depending upon the laid location of LCX when an information measured by an obstacle detection device was transmitted as a wireless signal to the other obstacle detection device. To describe this in further detail, if the LCX is laid on the surroundings of mountains or buildings and so forth, for example, there may be the possibility that the quality of communication is deteriorated because the wireless signal undergoes multiple reflections.
Accordingly, an object of the present invention is to obtain an obstacle detection device capable of precisely detecting the presence and position of an obstacle regardless of the whether the obstacle is motionless or moving, by utilizing the spread spectrum technology.
A further object of the present invention is to obtain an obstacle-detection in which the interference of signals for detecting an obstacle between adjacent obstacle detection devices is suppressed, a leakage transmission path is used to supply an electric source to the respective devices, and information is transmitted between devices, by utilizing spread spectrum technology.
The aforementioned objects can be achieved by an obstacle detection device, comprising: signal transmitting and receiving means for detecting an obstacle, comprising a first leakage transmission path on the transmitting side for radiating a signal for detection of an obstacle and a second leakage transmission path provided opposite to the first leakage transmission path and receiving the signal for detection of an obstacle; spread spectrum signal transmitting means connected to one end of the first leakage transmission path, for generating the spread spectrum signal for detecting an obstacle based on a spread code and causing the first leakage transmission path to radiate the spread spectrum signal for detection of an obstacle; and spread spectrum signal receiving means connected to one end of the second leakage transmission path, which end is the opposite to where the spectrum signal transmitting means is connected to the first leakage transmission path, wherein the spread spectrum signal receiving means further comprises: reference spread spectrum signal generation means for generating a reference spread spectrum signal based on a spread code of a code series identical to that used by the spread spectrum signal transmitting means; correlation means for calculating a level of correlation between the reference spread spectrum signal generated by the reference spread spectrum signal generation means and the spread spectrum signal for detection of an obstacle received by the second leakage transmission path, based on spread code phase-locked to the spread code of the spread spectrum signal for detection of an obstacle detected by the second leakage transmission path; and detection means for detecting an obstacle based on a change of a correlation level calculated by the correlation means.
The spread-spectrum signal receiving means may comprise time measurement means for measuring a spread spectrum signal propagation time period elapsing from the time when the spread spectrum signal transmitting means generates the spread spectrum signal for detection of an obstacle until the time when the correlation means is capable of calculating a correlation level of the both spread spectrum signals subsequent to generation, by the reference spectrum signal generation means, of the reference spread spectrum signal which is in synchronization with the spread code of the spread spectrum signal for detection of an obstacle, and the detection means may detect a position of an obstacle existing within an obstacle detection area formed by the signal transmitting and receiving means for detecting an obstacle, based on the spread spectrum signal propagation time period measured by the time measurement means.
The spread spectrum signal receiving means may have delaying means for delaying a phase of the spread code used by the reference spread spectrum signal generation means with respect to the spread spectrum signal for detection of an obstacle generated by the spread spectrum signal transmitting means by a desired time period.
The aforementioned objects can also be achieved by an obstacle detection system comprising: a plurality of obstacle detection devices according to at least one of claims 1 through 3, wherein adjacent devices generate spread spectrum signals based on different spread codes.
The aforementioned objects can also be achieved by an obstacle detection system comprising: a plurality of obstacle detection devices according to at least one of claims 1 through 3, wherein the obstacle detection devices are connected via leakage transmission paths, each of the obstacle detection devices comprising: synthesizing means for generating a superimposed signal by superimposing an electric power on the spread spectrum signal for detection of an obstacle and transmitting the superimposed signal to the adjacent device via the leakage transmission path, and isolating means for isolating the electric power and the spread spectrum signal from the superimposed signal generated by the synthesizing means, and supplying the electric power to the obstacle detection device.
The adjacent devices may generate spread spectrum signals based on different spread codes.
The obstacle detection devices may be connected via the leakage transmission paths and are provided with information transmitting and receiving means for transmission and reception of detection information between the obstacle detection devices.